Vapor Diffusion vs. Air Leakage vs. Bulk Transport

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Air movement as a moisture transport mechanism is typically far more important than vapor diffusion and bulk transport of moisture. As noted in the figure below the movement of water vapor through a 1-inch square hole via a typical air pressure differential is significantly greater than the movement of water vapor as a result of vapor diffusion and the transport of moisture via a rain event through a 32 sq.ft. sheet of gypsum sheathing.

In most climates, if the movement of moisture-laden air into a wall or building assembly is eliminated, movement of moisture by vapor diffusion is not likely to be significant. Keeping the building enclosure air tight is essential in keeping it dry.